In chroma keying, two scenes are electrically combined into a single composite picture in the following manner. A first scene consists of the elements of a foreground picture, typically including actors, in front of a background of a specified colour, usually blue. The second scene consists of a background picture, typically of a distant location. By use of chroma keying the foreground picture is made to appear in front of the background picture, thus the actors can be made to appear to be at the distant location. To do this a camera views the first scene and provides a first video signal. A second video signal is generated representative of the second scene by any suitable means such as a camera, telecine machine, or video tape recorder, synchronised with the camera viewing the first scene. From the first scene there is also produced a waveform, known as the keying signal, which indicates whether the first video signal instantaneously corresponds to a back-drop area of the first scene or a foreground area of the scene. This keying signal can then be used to operate a fast selector switch so as to select the first video signal when this represents foreground information, but at other times to select the second, background video signal, to provide the composite signal.
Chroma keying has typically operated with RGB signals, that is with non-encoded colour signals. However there is a trend in television studio equipment to operate on encoded luminance (Y), and colour difference (R-Y and B-Y) signals and to carry out all signal processing on these encoded components.